A tunneling diode is a type of semiconductor diode that is capable of femtosecond operation. This fast operation is made possible by the use of the quantum mechanical effect called tunneling. Metal-insulator-metal (MIM) tunneling diodes are micro-scale or nano-scale devices that include a very thin layer comprising one or more dielectric materials that is sandwiched between two metal electrodes having different work functions.
MIM tunneling diodes having a response time of less than a picosecond hold great promise to outperform their semiconductor counterparts (e.g., Schottky diodes) in mixing and detection of terahertz and infrared radiation. The most significant limitation on the speed of operation of an MIM tunneling diode is its junction capacitance. To a large extent, the junction capacitance of such a diode is determined by the size of the tunnel junction. Therefore, one method of reducing the capacitance of an MIM tunneling diode is to reduce the size of the diode's junction.
To achieve a response time in the femotosecond range, the junction needs to have a nanoscale size, in which case the area of the junction must be on the order of 0.01 μm2. While such small junction sizes have been achieved using e-beam lithography, e-beam lithography is a slow “direct-write” nanolithography method that is not suitable for low-cost, high-volume, production. In order to be able to manufacture MIM tunneling diodes in a commercial context, an alternative fabrication method is required.